In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
It is well known that for cemented carbide cutting tools used in metal machining, the wear resistance of the tool edge can be considerably increased by applying thin, hard surface layers of metal oxides, carbides or nitrides with the metal either selected from the transition metals from the groups IV, V and VI of the Periodic Table or from the group silicon, boron and aluminium. The coating thickness usually varies between 1 and 15 μm. The most widespread method for depositing such coatings is CVD (Chemical Vapor Deposition).
The practice of applying a pure ceramic layer such as alumina on top of layers of metal carbides and nitrides for further improvements of the performance of a cutting tool was recognized as is evidenced in Re 29,420 (Lindström et al) and U.S. Pat. No. 3,836,392 (Lux et al). Alumina coated cutting tools are further disclosed in U.S. Pat. No. 4,180,400 (Smith et al), U.S. Pat. No. 4,619,866 (Smith et al), U.S. Pat. No. 5,071,696 (Chatfield et al), U.S. Pat. No. 5,674,564 (Ljungberg et al) and U.S. Pat. No. 5,137,774 (Ruppi) wherein the Al2O3 layers comprise the α- and κ-phases and/or combinations thereof. For example, in U.S. Pat. No. 4,180,400, an alumina deposition process is disclosed where tetravalent ions of, e.g., Ti, Zr or Hf are added in their halide compositions to the reaction gas mixture in order to deposit essentially a single phase κ-Al2O3.
The practice of mixing different metal halides in order to deposit composite ceramic coatings is evidenced in U.S. Pat. No. 4,701,384 (Sarin et al), U.S. Pat. No. 4,745,010 (Sarin et al) and U.S. Pat. No. 5,827,570 (Russell) where processes for depositing mixtures of, e.g., Al2O3 and ZrO2 are described.
In further efforts to improve the cutting performance of alumina coated cemented carbide cutting tools, particularly in the machining of nodular cast iron, deposition processes yielding fine-grained, single phase α-Al2O3 exhibiting specific crystal orientation (texture) and surface finish are disclosed in U.S. Pat. No. 5,654,035 (Ljungberg et al), U.S. Pat. No. 5,766,782 (Ljungberg), U.S. Pat. No. 5,834,061 (Ljungberg) and U.S. Pat. No. 5,980,988 (Ljungberg).
However, the machining of nodular cast iron is still considered to be a demanding metal working operation. This is particularly obvious in heavy and interrupted machining operations where an α-Al2O3 coated tool often suffers from extensive flaking of the alumina layer from the tool substrate.